The mechanisms that dictate the final program of gene expression in a fully differentiated cell can be revealed by starting at either end of the regulatory cascade. To examine the series of controls operating on cells of the oligodendrocyte lineage, we have cloned a number of putative transcription factors that recognize one of the final targets of regulation in myelinating glial cells, proteolipid protein (PLP). One such factor named MyTI (myelin transcription factor I) is a novel member of the zinc finger superfamily. Several observations suggest that MyTI may be instrumental in early stages of oligodendrocyte development and myelin production. MyTI message is more highly expressed in oligodendrocyte progenitors than in differentiated oligodendrocytes. In progenitors, MyTI immunoreactivity appears as speckles within the nucleus, suggestive of an association of MyTI with spatially segregated functional domains. MyTI has two clusters of DNA-binding domains, one of which binds to a consensus site represented several times in the PLP promoter and also found in other myelin genes. MyTI may represent an emerging class of regulatory proteins with a combination of features that predicts a role in coordinating the expression of a set of genes. Characteristic features shared by such proteins would include a structure of multiple DNA-binding domains which are each stabilized by a divalent cation, initial expression that markedly precedes the target gene, and localization within discrete macromolecular compartments of the nucleus. The isolation of clones encoding transcriptional regulatory proteins permits a search for the growth factors and other cytokines that are critical to the initiation and maintenance of myelin gene transcription during development and regeneration. We have evidence for the molecular mechanism by which thyroid hormone affects the transcription of the PLP gene and thereby has an impact on myelination. A site for the thyroid hormone receptor (THR) was identified within the PLP gene and shown to be essential for hormonal responsiveness. A prerequisite for THR activation of this site is the dimerization of THR with another member of the nuclear hormone receptor family, PPAR, which is a regulator of lipid metabolism. By employing a family of transcription factors whose heterodimerization specificity determines the target specificity and ultimately allows the coupling of divergent signaling pathways, an oligodendrocyte may balance the requirements for myelin protein synthesis with those of myelin lipid synthesis.